1. Field of the Invention
The present invention is directed to an optical fiber strain sensor and, more particularly, to an optical fiber strain sensor which can measure strain at discrete locations anywhere on the surface of a structure.
2. Description of the Related Art
A known technique for measuring strain is to use the light transmitted by an optical fiber affixed to an object under strain. Conventionally, the modal pattern produced by light at the receiving end of an optical fiber under strain is monitored to detect changes in the modal pattern. The changes in the pattern are caused by changes in the optical path length due to changes in the index of refraction and the propagation constants of each fiber mode. As a result, the modal phase term, .beta..sub.In z, of the electric fields is shifted by an amount .DELTA..PHI.. This phase shift phenomenon has been theoretically studied and experimentally used in applications that involve determination of strain using single mode, few mode and multimode fibers.
A single mode fiber is the simplest kind of strain sensor using optical fibers. It is basically an interferometer that compares the modal phase shift of two fibers: one fiber is subjected to strain and the other serves as a reference. Such a device is described in "Fiber Optics Strain Gauge" by Butter et al. in Applied Optics, vol. 17, pages 2867-2869, September 1978, for surface mounted sensors using "weakly guiding" optical fibers. Another example of strain sensors using weakly guiding optical fibers was described in "Complete Phase-Strain Model for Structurally Embedded Interferometric Optical Fiber Sensors" by Sirkis et al. in the Journal of Intelligent Material Systems and Structures, vol. 2, pages 3-24 (1991). The latter paper describes both surface mounted and embedded sensors in many strain field configurations.
There are several drawbacks to optical fiber strain sensors using known techniques. First, the phenomenon has only begun to be investigated and the theoretical underpinnings are not fully understood. Second, for multi-mode fibers the shifting of patterns can be complex and requires either an intelligent detector, or careful alignment of a detector with a pattern produced for a given amount of strain. Third, strain is measured over the entire length of the optical fiber with the result that it is difficult to mount an optical strain sensor to measure strain at a discrete location.